Synthetic composite materials possess desirably high strength-to-weight ratios; however, composites typically lack dynamic functionality that occurs in natural composite materials. Natural composite materials, for example, rely on pervasive vascular networks to enable a variety of biological functions, in both soft and hard tissue. Composite structures such as bone tissue or wood are lightweight and have high strength, yet contain extensive vasculature capable of transporting mass and energy.
An ongoing challenge in materials science is the development of microvascular networks in synthetic composites, where the composite materials may be formed using conventional composite manufacturing processes. Specialized fabrication methods such as laser-micromachining, soft lithography, templating with degradable sugar fibers, and incorporating hollow glass or polymeric fibers can produce some microvascular structures in composite materials. These specialized methods, however, are not currently suitable for rapid, large-scale production of fiber-reinforced composites with complex vasculatures.
In one approach to microfluidic composites, relatively short microfluidic channels are provided in a matrix in the form of hollow glass fibers (WO 2007/005657 to Dry). The glass fibers are present as repair conduits containing a fluid that can heal a crack in the composite matrix. A significant limitation of this approach is the brittle nature of the hollow glass fibers, which limits the shapes and lengths of microfluidic channels that can be present in the composite. In addition, the glass fibers cannot readily be used to form a microfluidic network.
In another approach to microfluidic composites, microfluidic channels are formed in a polymeric matrix by arranging hollow polymeric fibers and then forming the matrix around the hollow polymeric fibers (U.S. Publication No. 2008/0003433 to Mikami). Hollow polymeric fibers may offer a wider variety of microfluidic channel shapes than those available from hollow glass fibers. This approach, however, also has a number of limitations, including an inability to form a network from the individual hollow fibers, the relatively small number of materials available as hollow fibers, and the possibility of incompatibility between the hollow fiber and the matrix and/or between the hollow fiber and substances introduced into the channels.
Microfluidic networks can be formed in a polymeric matrix using a three-dimensional (3-D) direct-write assembly technique (U.S. Publication No. 2008/0305343 to Toohey et al.). While this fabrication method provides excellent spatial control, the resulting networks typically will not survive the mechanical and/or thermal stresses encountered in the conventional processes of forming reinforced composites.
Fabrication of microvascular composites using sacrificial fibers is described in U.S. Publication No. 2013/0065042 to Esser-Kahn et al., where the construction of microvascular networks within a fiber-reinforced composite by vaporization of sacrificial fibers is taught. The pre-processing of sacrificial fibers involves solvent assisted diffusion of catalyst particles into poly(lactic acid) fibers followed by solvent evaporation, or the wet spinning of mixtures of poly(lactic acid) and catalysts.
However, composites fabricated using this method have limitations in terms of flexibility to form interconnections in a repeatable and reliable manner as the interconnections using sacrificial fibers are mainly formed by creating physical contacts between fibers or solvent welding of fibers (see FIGS. 31A-E of U.S. Publ. No. 2013/0065042). As such, connecting microfluidic channels with in-plane interconnections to form complex in-plane architectures is not possible. As a result, the resulting microchannels lack spatial interconnectivity, thus limiting fluidic pathway redundancy.
Moreover, the solvent assisted diffusion of catalyst particles into poly(lactic acid) is time consuming, taking an average of fifty hours for infiltrating the catalyst into the poly(lactic acid). The evacuation of the products resulting from the vaporization of the sacrificial fibers can also be inconsistent and in some cases incomplete due to blockages from residual catalyst particle agglomerations within the micro-channels.